1. Field of the Invention
The present invention relates to a process for reducing the cyanide content of a solution.
The use of an alkaline aqueous solution of a cyanide salt for recovery of gold and/or silver values from ores, tailings and wastes has been well known and documented for along period. While this process is well known and widely used for such precious metal recovery it is also to be understood that the waste water streams from such processes contain cyanide in solution which could be dangerous if allowed to enter the environment and particularly the water supply.
For this reason, waste aqueous process streams from gold treatment plants using alkaline cyanide solutions as the leachant must meet specific composition limits before they are discharged into the environment. In particular, the total cyanide and free cyanide concentrations must be below the maximum limits set by local, state and federal licensing authorities.
Similar requirements are imposed on the effluent discharged from cyanide production plants and from other industrial processes some of which produce waste water streams containing other species notably ammonia as well as cyanide ions.
2. Discussion of the Prior Art
A number of processes have been proposed to bring about the chemical breakdown of the total and free cyanide components of typical effluent streams. Some of the processes have been patented and several are presently used on the commercial scale.
High temperature hydrolysis brings about the total destruction of the cyanide ion, but the high temperatures and extended retention times required makes this process inefficient in cost terms. Some chemical reagents convert the cyanide ion only to cyanate. In many localities, discharge of cyanide is not permitted. Further destruction of cyanate may be inefficient or costly in reagent consumption terms, or in fact require a different chemical reagent.
The complexity of cyanide destruction is compounded by the presence of other species in solution that interfere with the destruction mechanism and/or substantially increase the capital and operating costs. One such species is dissolved ammonia.
Treatment of an effluent or processing liquor containing, for example, an equivalent concentration of cyanide ions and dissolved ammonia, by the standard cyanide destruction methods is unsatisfactory in terms of additional reagent consumption, increased capital and operating costs and/or incomplete cyanide destruction. These methods include the use of hydrogen peroxide, chlorine, chlorine/calcium hydroxide, hypochlorite, or sulphur dioxide.
The applicants are aware, for example, of a proposal described in Australian patent application No. 73261/81 (PCT International Publication Number W082/00288) for removing cyanide ions from solutions. This process provides for adjusting the solution of a pH to at least about 11 and to a halide content at least equivalent to the cyanide concentration. Electrolysis is performed using intense agitation of the electrolyte to result in deposition of solid carbon on the anode. Electrolytic methods generally require a readily available source of cheap electricity for economic operation and this is not always available in remote areas. Further, this proposal does not deal with any dissolved ammonia also present in the waste water stream.
Australian patent application No. 17053/88 (PCT International Publication Number WO 88/04408) relates to a method for separating by aeration a component such as cyanide from a liquid containing the component. The method comprises passing the liquid through an array of aeration columns arranged in stages so that the liquid flowing from one column in a first stage is divided into two or more streams which are introduced into separate aeration columns in a successive second stage. However this process is likewise unsuitable for use in remote areas and does not effectively deal with any dissolved ammonia present in the waste water system.
Australian patent application No. 76929/81 relates to the chlorination of waste water containing ammonia, cyanide and phenol. This method involves adding a chlorinating compound to the waste water in a first vessel at pH 8.5 to 10, passing the waste water to a second vessel at pH 6 to 8 and controlling the addition of the chlorinating compound in response to the redox potential in the second vessel so as to maintain the redox potential at +625 millivolts to +750 millivolts. This method requires close monitoring of redox potentials and is specifically directed at waste water containing phenol as well as ammonia and cyanide contaminants.